1. Technical Field
The present invention relates to semiconductor processing plasma reactor magnets for use in semiconductor fabrication process chambers employing magnetically-enhanced plasmas. More particularly, the invention relates to magnets for such chambers having a precisely adjustable magnetic field which enhances the instantaneous uniformity of ion bombardment of a semiconductor workpiece with either a nearly perfectly uniform magnetic field across the workpiece or a magnetic field with a nearly perfectly uniform gradient across the workpiece or another precisely adjustable desired magnetic field distribution.
2. Background Art
Plasma etch chambers are widely used to provide suitable etching environments for production of wafers for use in integrated circuits. Semiconductor fabrication process chambers commonly employ plasmas to enhance the performance of various processes for fabricating semiconductor devices on silicon wafers or other workpieces. Such processes include sputter etching, plasma-enhanced chemical etching, plasma enhanced chemical vapor deposition, and ionized sputter deposition. The high energy level of reagents in the plasma generally increases the rate of the fabrication process, and often reduces the temperature at which the semiconductor workpiece must be maintained to perform the process.
As integrated circuits geometries shrink, plasma etch processes must enable higher resolution patterns. Presently, the most widely used dry etch technique, reactive ion etching (RIE), offers directionality and selectivity together with high throughput. Magnetic enhanced reactive ion etching (MERIE) offers increased manipulation of the plasma for enhanced wafer etch control.
In a typical MERIE reactor, four coils, each having numerous turns, formed in loops having the rough shape of a bay windowframe, are positioned azimuthally around the plasma etch chamber. Coils are positioned such that the wafer plane is centered at the vertical midplane of the four-coil system. The coils are energized independently, in time-dependent waveforms; energization of the four coils is synchronized chronized for optimal plasma etch chamber magnetic field distribution and temporal rotation. The desired magnetic field distribution at and near the wafer plane is partially provided by the vertically oriented portions of the coil loops, which cause location-dependent (r,.theta.) field components in both x- and y-horizontal directions, but none in the z-vertical (axial) direction. The curved horizontal portions of the coil loops, necessary to complete electrical circuits, individually provide additional field components in all three coordinate directions. Undesired field z-components from upper and lower loop segments cancel at magnet's vertical midplane, where the wafer is optimally positioned, leaving a generally radially-directed field component remaining at the wafer plane.
Magnetically-enhanced plasma chambers employ magnetic fields to increase the density of charged particles in the plasma, thereby further increasing the rate of the plasma-enhanced fabrication process. Increasing the process rate is highly advantageous because the cost of fabricating semiconductor devices is directly proportional to the time required for fabrication.
Despite this advantage, many plasma chambers in commercial use do not employ magnetic enhancement, because magnetic enhancement has been found to increase the likelihood of damaging the semiconductor devices on the wafer. (See Fang McVittie, "Charging damage to gate oxides in an O.sub.2 magnetron plasma", J. Appl. Phys., vol. 72, no. 10, pp. 4865-4872, Nov. 15, 1992; and Fang McVittie, "The role of `antenna` structure on thin oxide damage from plasma induced wafer charging", Mat. Res. Soc. Symp. Proc., vol. 265, pp. 231-236, 1992.) Therefore, a need exists for a magnetically-enhanced plasma chamber that affords the benefits of conventional magnetic enhancement but with a reduced risk of semiconductor device damage.
It is believed that a major cause of semiconductor device damage in conventional magnetically-enhanced plasma chambers may be that the uniform magnetic field conventionally considered ideal by some workers actually causes an E.times.B drift of electrons in the plasma pre-sheath, which in turn causes the ion flux bombarding the semiconductor wafer or workpiece to have a highly non-uniform distribution across the surface area of the workpiece.
The reasons for instantaneous non-uniformity of ion flux in conventional magnetically-enhanced plasma chambers will be explained. FIG. 1A shows a magnetically-enhanced plasma chamber suitable for either etching or chemical vapor deposition (CVD). The vacuum chamber is enclosed by cylindrical side wall 12, circular bottom wall 14, and circular top wall or lid 16. The lid 16 and bottom wall 14 may be either dielectric or metal. An electrically grounded anode electrode 18 is mounted at the bottom of the lid 16. The anode electrode may be perforated to function as a gas inlet through which process gases enter the chamber. The side wall 12 may be either dielectric or metal. If it is metal, the metal must be a non-magnetic material such as anodized aluminum so as to not interfere with the magnetic field created by electromagnet coils outside the chamber. If the side wall is metal, it will function as part of the anode.
The semiconductor wafer or workpiece 20 is mounted on a cathode electrode 22, which, in turn, is mounted in the lower end of the chamber. A vacuum pump, not shown, exhausts gases from the chamber through exhaust manifold 23 and maintains the total gas pressure in the chamber at a level low enough to facilitate creation of a plasma, typically in the range of 10 millitorr to 20 torr, with pressures at the lower and higher ends of the range being typical for etching and CVD processes, respectively.
An RF power supply 24 is connected to the cathode electrode 22 through a series coupling capacitor 26. The RF power supply provides an RF voltage between the cathode electrode and the grounded anode electrode 18 which excites the gases within the chamber into a plasma state. The plasma body has a time-average positive DC potential or voltage relative to the cathode or anode electrodes which accelerates ionized process gas constituents to bombard the cathode and anode electrodes.
Magnetic enhancement of the plasma most commonly is implemented by a DC magnetic field in the region between the cathode and anode electrodes. The direction of the magnetic field is transverse to the longitudinal axis of the chamber, i.e., transverse to the axis extending between the cathode and anode electrodes. Various arrangements of permanent magnets or electromagnets are conventionally used to provide such a transverse magnetic field. One such arrangement is the pair of coils 30 shown in FIG. 1A, disposed on opposite sides of the cylindrical chamber side wall 12. The two coils 30 are connected in series and in phase to a DC power supply, not shown, so that they produce transverse magnetic fields which are additive in the region between the two coils. This transverse magnetic field is represented in FIGS. 1A and 1B by the vector B oriented along the negative x-axis. In the preferred embodiment, the diameter of each coil approximately equals the spacing between the two coils. (The second pair of coils 32 shown in FIG. 1B will be discussed later, but may be ignored for purposes of the present discussion.) An example of such a magnetically enhanced plasma etch chamber is described in commonly assigned U.S. Pat. No. 5,215,619 issued Jun. 1, 1993 to Cheng et al., the disclosure of which is hereby incorporated into the present patent specification.
Because the plasma body has a positive time-average potential or voltage relative to the cathode electrode 22, the time-average electric field E in the plasma pre-sheath adjacent the cathode will be directed downward from the plasma toward the cathode, thereby giving the free electrons in the pre-sheath a drift velocity vector v whose time-averaged value is oriented upward toward the plasma body, as represented by the vector v.sub.e in FIG. 1A. In response to the DC magnetic field vector B, these free electrons of charge e and velocity v additionally experience an ev.times.B force--commonly called an E.times.B drift--whose time-averaged value is approximately coplanar with the semiconductor wafer 20 and orthogonal to the magnetic field vector B, as illustrated in FIG. 1B by the E.times.B vector oriented along the y-axis. In this context, the term "time average" means averaged over one period of the RF frequency or frequencies at which the plasma is excited, this period typically being less than 10.sup.-7 second. This time average over one RF period is unrelated to the time averaging due to optional rotation of the workpiece relative to the magnetic field, which typically has a rotation period on the order of 0.1 to 2 seconds.
This E.times.B drift of free electrons is believed to be a major source of semiconductor device damage in conventional magnetically-enhanced plasma chambers. Specifically, it is believed that the E.times.B drift unevenly distributes the free electrons in the plasma pre-sheath, so that the electron concentration progresses from minimum to maximum from the negative y-axis to the positive y-axis. In other words, the electron concentration in the pre-sheath is lowest near the 6 o'clock (270.degree. azimuth) position of the wafer 20, and is highest near the 12 o'clock (90.degree. azimuth) position of the wafer. A high electron concentration produces a high ion concentration, hence the flux of ions bombarding the wafer is lowest and highest at the 6 o'clock and 12 o'clock positions of the wafer, respectively. It is believed this spatial non-uniformity of ion flux bombarding the wafer produces electrical currents in the wafer which often damage the semiconductor devices on the wafer.
Although the present invention does not depend on this theory, it is believed the mechanism of damage due to ion flux instantaneous spatial non-uniformity is as follows. An ion flux spatial non uniformity causes spatially non-uniform accumulation of electrical charge on the wafer. The differential in electrical charge produces voltages and current flow between different points on the wafer. If the voltage across any of the dielectric structures fabricated on the wafer exceeds the maximum safe voltage of that dielectric structure, then a current will flow through the structure which is likely to damage the dielectric structure and therefore damage one or more semiconductor devices on the wafer. We believe a more uniform spatial distribution of instantaneous ion flux bombarding the wafer would reduce the likelihood of such damage.
Conventional magnetically-enhanced plasma chambers attempt to ameliorate this non uniformity by slowly rotating the magnetic field relative to the wafer, typically at a rotation frequency in the range of one-half to five rotations per second. In some designs, the wafer 20 or the magnet 30 is physically rotated. In other designs, as illustrated in FIG. 1B, the rotation is performed electronically by providing a second pair of coils 32 orthogonal to the first pair of coils 30. The magnetic field can be rotated in 90.degree. increments by successively and periodically connecting the DC power supply: (1) to the first coil pair 30 with positive polarity; (2) to the second coil pair 32 with positive polarity; (3) to the first coil pair 30 with negative polarity; and (4) to the second coil pair 32 with negative polarity. Alternatively, the magnetic field can be rotated continuously by replacing the DC power supply with a very low frequency (in the range of 0.1 to 10 Hz) power supply having quadrature outputs connected to provide current to the first coil pair 30 offset in phase by 90.degree. from the current provided to the second coil pair 32.
Rotating the magnetic field relative to the wafer greatly reduces time-average spatial non uniformity in the ion flux bombarding the wafer, and therefore can provide acceptable spatial uniformity of etch rate (in an etching chamber) or deposition rate (in a CVD chamber) on the wafer surface. However ever, rotating the magnetic field does not in any way improve the instantaneous spatial uniformity of ion flux on the wafer surface, and therefore does not solve the problem of semi-conductor device damage in magnetically-enhanced plasma chambers. Consequently, magnetic enhancement heretofore has been used only in a small fraction of commercial plasma chambers for semiconductor fabrication.
One approach to improving the instantaneous spatial uniformity of ion flux on the wafer surface is to provide a magnet netic field which is weakest in the region of the plasma pre-sheath to which electrons will be swept by the E.times.B drift, and which is strongest in the region from which electrons will be swept by the E.times.B drift. Stated more mathematically, the approach is to provide a magnetic field whose magnitude at points in the plane of the wafer is characterized by a gradient vector pointing in the opposite direction of--i.e., oriented approximately 180.degree. from--the projection of the E.times.B vector in the plane of the wafer. Heretofore, it has not been possible or practical to provide a magnetic field having this exact orientation or distribution over the entire surface of the wafer, so that the efficacy of this approach has necessarily been limited. However, some advantage is realized if only the magnetic field has at least approximately this orientation or distribution over a majority of the surface of the wafer.
Regions of the plasma having a higher magnetic field magnitude will have a higher density of free electrons. By providing the strongest magnetic field magnitude in the region of the plasma pre-sheath where the E.times.B drift will tend to deplete the electron concentration, and by providing the weakest magnetic field where the E.times.B drift will tend to augment the electron concentration, the foregoing approach tends to equalize (to a limited degree) the electron concentration throughout the pre-sheath adjacent the wafer. A more spatially uniform electron concentration in the pre-sheath sheath produces a more spatially uniform flux of ions bombarding the wafer. The foregoing approach improves the instantaneous spatial uniformity of ions bombarding the wafer, and thereby reduces the risk of damage to semiconductor devices on the wafer.
FIG. 2A illustrates one embodiment of the foregoing approach as described in co-pending U.S. application Serial No. filed by entitled "Magnetically-Enhanced Plasma Chamber with Non-Uniform Magnetic Field". In FIG. 2A, two adjacent, mutually orthogonal annular coils to generate the magnetic field. The two coils are identical and connected together in series or, as illustrated in the figure, in parallel. The series or parallel combination is connected to a DC power supply 60. Preferably, the output current of the DC power supply 60 is controlled by a conventional microcontroller or micro-processor 65 to facilitate adjustment of the magnetic field strength by a human operator.
The first coil 40 is located at the 9 o'clock side (180.degree. azimuth) of the wafer 20, is oriented perpendicular to the x-axis, and is connected to the DC power supply with a polarity which produces a magnetic field oriented along the x-axis, i.e., in the positive x-direction or the 0.degree. azimuth direction.
The second coil 42 is located at the 12 o'clock side (90.degree. azimuth) of the wafer 20, is oriented perpendicular to the y-axis, and is connected to the DC power supply 60 with a polarity which produces a magnetic field oriented along the y-axis, i.e., in the positive y-direction or the 90.degree. azimuth direction. (To simplify the drawing, the chamber wall 12 surrounding the wafer is omitted from FIG. 2A and subsequent figures.)
The two coils produce a magnetic field in a region just above the wafer surface (specifically, in the region that includes the plasma sheath and pre-sheath adjacent the wafer) which is counterclockwise when viewed from above the wafer, looking down toward the wafer surface. In FIG. 2A the counterclockwise magnetic field pattern is represented by circular arcs 44a, 44b, 44c, 44d terminating in arrowheads pointing toward the second coil 42. Above the surface of the wafer 20, the magnetic field strength is greatest at the point P on the perimeter of the wafer at a 135.degree. azimuth because the two coils are closest to each other at that point. The magnetic field is weakest at the point Q on the perimeter of the wafer at a 315.degree. azimuth because the two coils are farthest from each other at that point. Another way of defining points P and Q is that they are the points on the wafer surface which are nearest and farthest, respectively, from the vertex "V" of the angle between the two electromagnet coils 40 and 42.
The counterclockwise magnetic field pattern produces an E.times.B drift of the free electrons in the plasma pre-sheath in a direction orthogonal to the field lines, as illustrated by the E.times.B vectors in FIG. 2A. More specifically, the E.times.B vectors at different points over the wafer surface point in different directions, but they always point away from the highest magnitude magnetic field line 44a and toward the lowest (magnitude magnetic field line 44d. For example, the E.times.B drift of the free electrons is away from the point P having the highest magnetic field strength and toward the point Q having the lowest magnetic field strength.
The spatial variation in magnetic field strength tends to produce the highest electron concentration in the pre-sheath near point P, and the lowest electron concentration in the pre-sheath near point Q. However, the orientation of the E.times.B vectors tends to produce the opposite result, i.e., the lowest and highest electron concentrations in the pre-sheath near points P and Q, respectively. Therefore, the effects of magnetic field strength variation and E.times.B drift tend to offset each other, thereby producing a more spatially uniform instantaneous distribution of free electrons in the pre-sheath adjacent the wafer surface.
This more spatially uniform instantaneous distribution of free electrons in the pre-sheath produces a more uniform instantaneous spatial distribution of ion flux bombarding the wafer, thereby reducing the risk of damage to semiconductor devices on the wafer.
As in conventional designs, the time-averaged uniformity of etch rate or deposition rate, in the case of etch chambers or CVD chambers, respectively, can be further improved by slowly rotating the two magnets relative to the wafer, or by rotating the wafer relative to the magnets. The rotation can be performed electrically, rather than mechanically, by providing an array of electromagnets encircling the workpiece, and powering the electromagnets in a sequence which causes the magnetic field to rotate around the center of the workpiece. This electrical rotation method is described in detail in the above-referenced co-pending application.
The two electromagnets are orthogonal in the case of FIG. 2A because such a geometry permits the electromagnets to be positioned as close as possible to a circular semiconductor wafer 20. However, a similar magnetic field pattern will be produced even if the angle between the two electromagnets is changed significantly. For example, FIG. 2B illustrates a modification of the design of FIG. 2A in which the electromagnetic coils 40, 42 are oriented at 180 degrees relative to one another.
Magnetic field distribution at a given moment in time from a typical energization scheme of the conventional MERIE four-coil system typically exhibits azimuthal nonuniformities in both magnitude and gradient. (Even greater nonuniformities are exhibited in a two-coil system.) Azimuthal nonuniformities may be partially averaged-out over time via a synchronized rotation of the energization of the four coils. Still, due to the four fold symmetry, complete azimuthal averaging is not possible, nor is the manipulation or homogenization of radial nonuniformities. As a result, conventional magnets cannot provide precise compensation for the E.times.B drift, and therefore cannot provide the optimum instantaneous ion flux distribution uniformity. In fact, it has been observed that such conventional magnets produce fields having as much as a 40% nonuniformity in either magnitude or gradient, so that compensation for E.times.B drift is inexact. Thus, the risk of device damage due to charge buildup remains a significant problem to be solved.
Another disadvantage of conventional magnets in MERIE reactors is that such magnets provide a predetermined field pattern, which may be rotated about the axis of the wafer but whose shape or distribution is otherwise not adjustable or controllable. Thus, the magnet must be designed to provide the best compensation for E.times.B drift possible for a particular reactor under a particular set of conditions. Any departure from those conditions may impair the compensation for E.times.B drift already impaired by the magnet's individually ability to provide a uniform field or gradient within a 40% variation. Moreover, any required corrections discovered in the production environment by the user cannot be implemented without physically modifying the magnet, a costly and time-consuming procedure.